It is well known that polyurethane elastomers and sealants can be manufactured by reacting an organic polyisocyanate with a high molecular weight polyol, generally in a range of 2,000 MW to 20,000 MW (equivalent weights of generally 900 to 10,000). Polyoxypropylene polyols prepared with alkali metal hydroxide catalysts, such as NaOH or KOH, begin to show a high degree of unsaturation as the molecular weight of the polyol increases. Polyoxypropylene polyether polyols generally begin forming unsaturation groups when the hydroxyl number of the polyol is about 60 or less, or the equivalent weight of the compound is about 900 or more. When using the conventional alkali metal hydroxide catalyst for the preparation or high molecular weight polyoxypropylene polyether polyols, the degree of unsaturation is generally 0.10 meq/g of polyol or more.
A polyol having a high degree of unsaturation means that a significant number of monools are present among the polyols. A monool undergoing reaction terminates further chain growth; therefore, the properties of the polyurethane sealant or elastomer suffer. In particular, the tensile strength at break, the modulus of elasticity, the elongation, hardness, wear resistance, etc. begin to degrade.
In response to the above-mentioned problems, it has been proposed to lower the unsaturation of the polyols using double metal cyanide catalysts. For example, U.S. Pat. Nos. 5,096,993; 5,185,420; 4,985,491 to Olin Corporation; and WO 92/06139 disclose a preparation of polyether polyols having unsaturation levels of less than 0.04 meq/g prepared by double metal cyanide catalysts. The object of the inventions disclosed in these patents was to increase the physical properties of the elastomer, specifically, the tensile strength, including the modulus of elasticity and elongation, as much as possible. In each of the examples disclosed in these patents, polyether polyols prepared using double metal cyanide catalysts were reacted with isocyanates to produce both soft and hard elastomers. The polyols having an unsaturation level of 0.04 or less successfully increased the elongation percent, ultimate tensile strength, and the modulus of elasticity of the polyurethane elastomers and sealants prepared therewith.
Japanese patent application Kokai No. 2-263819 also teaches a preparation of polyurethane elastomers using polyols having hydroxyl numbers of 34 to 60 and a total degree of unsaturation of 0.03 meq/g or less. The catalysts proposed for the preparation of such low unsaturation polyols were diethyl zinc, iron chloride, metallic polyphyrin, and metallic cyanide complexes. Likewise, JP patent application Kokai No. 5-295073 and 2-263818 each propose employing a polyether polyol having a degree of unsaturation of 0.07 or less, the particular degree of unsaturation varying with the hydroxyl number of the polyol, also prepared using the same aforementioned catalysts. In each of the working examples of these Japanese references, polyols having unsaturation levels of less than about 0.03 were employed in the preparation of elastomers having high elongations, e.g., 700 percent or more; high ultimate tensile strengths of 1,000 psi or more; and a high modulus of elasticity (tensile strength at 100 percent elongation) of 500 psi or more.
While low unsaturation polyols prepared by double metal cyanide catalysts impart improved properties to polyurethane elastomers, such catalysts are not currently commercially available and are both time consuming and expensive to make. Further, double metal cyanide catalysts, if left in the polyol mixture, act as weak Lewis acids and affect the reactivity of the polyurethane foaming mixture. Other catalysts such as iron chloride and BF.sub.4 require temperatures generally of about 100.degree. C. to catalyze the reaction between the alkylene oxide and the initiator in the formation of the polyol. Furthermore, iron chloride tends to color the polyol. Thus, it would be desirable to employ a catalyst that can be used to catalyze the alkene oxide/initiator reaction at low temperatures, which can be removed from the polyol mixture without difficulty, or left in the polyol and neutralized, and which is an inexpensive alternative to double metal cyanide catalysts.
As mentioned above, the low unsaturation polyols markedly improve all physical properties of the polyurethane elastomers and sealants, including the modulus of elasticity. However, it would be desirable to make a polyurethane sealant or elastomer having a relatively low modulus of elasticity so that the material can expand or contract with changes of temperature.
We have also found that it would be highly desirable to manufacture a sealant or an elastomer which exhibits an increasing modulus of elasticity when strained between 100% and 300%. The toughness of such a sealant or elastomer would increase as it expands under adverse weather of mechanical conditions. We have found that sealants made with polyols manufactured using a double metal cyanide catalysts at unsaturation levels of less than 0.04 can exhibit a decreasing modulus of elasticity when subjected to strains at 100% and 300%. While none of the above described publications address this problem or suggest the desirability of this feature, EP 0573206 does advocate the use of low unsaturation polyols in order to decrease the modulus of elasticity when strained at 100% and 200%. Contrary to the objectives of EP 0573206, we desire to use a polyol in one of the embodiments which will increase the modulus of elasticity of a sealant when strained between 100% and 300%, while simultaneously imparting to the sealant a high elongation and satisfactory tensile strength at break.